Certain infectious plant viruses have a newly discovered capacity to spread within plants faster and more efficiently than previously imagined, possibly usurping a transport system used by healthy plants to transport large molecules needed for normal plant growth, according to University of California, Davis, researchers. In a cover article in the March 11 issue of the journal Cell, the researchers report seeing for the first time the movement of large viral molecules from living cell to living cell within a plant leaf. Eventually, the results may help researchers control viruses and improve crop production. Scientists have known for decades that plant viruses travel through transport channels known as "plasmodesmata," which link the insides, or cytoplasm, of each cell of the plant. Until now, however, it was thought that the plasmodesmata of a healthy plant only transported small nutritional molecules essential for plant growth. Scientists have wondered how large viral molecules fit through these small passageways, akin to trying to squeeze a bowling ball through surgical tubing. "These results show that plasmodesmata must have an ongoing escalator belt that trafficks large molecules from cell to cell; it is unique to plants," says co-author and plant physiologist William Lucas, a UC Davis professor of plant biology who has studied the plasmodesmata extensively. "There is no other way known to send a large molecule around the plant and into another cell." The new study describes how plant viruses may hitch a ride on this cell-to-cell transport system by producing certain proteins, called movement proteins, that may mimic those normally used by the plant to transport large molecules that regulate basic processes important for plant growth and reproduction. "Now that we understand what viral movement proteins are doing, we can try to block the ability of the virus to move within the plant," said co-author and virologist Robert Gilbertson, an assistant professor of plant pathology. "It would be a nice way to make virus-resistant plants." The paper provides the first direct evidence in living cells that viral movement proteins actually spread together with the viral genetic information, widening the channel to enable the large molecules to travel. The results also suggest that the plasmodesmata of uninfected plants may normally transport larger and more diverse molecules than previously believed. The study of viral movement provides a tool for probing the plasmodesmata functions of healthy plants. Using a type of geminivirus -- a unique family of viruses so named because of their submicroscopic resemblance to a pair of soccer balls stuck together -- the researchers identified a movement protein that widens the plasmosdesmata channel large enough for the virus to move easily into neighboring and distant cells. They also identified a second movement protein that exports newly replicated genetic information out of the cell nucleus. They found that the geminivirus has a two-part movement system. One protein returns the viral genetic information from the nucleus and another moves it across the cellular boundary. First identified in the late 1970s, geminiviruses have become among the most economically important crop plant viruses. Endemic to tropical and subtropical climates, these viruses are carried from plant to plant by tiny insects called whiteflies. Whiteflies and geminiviruses are causing crop losses in Florida, Texas and Southern California. This study began by showing that the suspected geminivirus movement protein, called BL1, could widen the plasmodesmata channel. The researchers harnessed the common bacteria E.coli to produce large quantities of the suspected cell-to-cell movement protein. Once purified, both the BL1 protein and colorful tracking molecules of various sizes were microinjected into a cell of a living plant leaf. Under a microscope, researchers found molecules up to 10 times larger than the normal width of plasmodesmata moving rapidly from cell to cell, presumably via the enlarged channel. Next, plant pathology graduate student Amine Noueiry figured out a way to fluorescently label the BL1 protein itself. Once injected into a cell, the protein, which is 30 times larger than the normal plasmodesmata channel, quickly moved out of the injected cell and into neighboring cells. Noueiry then highlighted the genetic material of the virus (DNA) and injected both the fluorescent genes and unlabeled movement protein into a living plant leaf. The movement protein enabled the genetic material also to move from cell to cell through the plasmodesmata. Finally, Noueiry teased out the function of the second movement protein after he noticed that the fluorescently labeled viral DNA naturally drifted into and lit up the cell nucleus. When he injected the second movement protein, which also seemed to be necessary for the virus to spread through the plant, the DNA moved out of the nucleus and back into the cell. This second movement protein, known as BR1, is a "nuclear export factor" and works to release the genetic information from the nucleus into the surrounding cytoplasm of the cell, where the BL1 protein then moves the virus DNA from cell to cell. The discovery that relatively large viral molecules can travel through a system once believed to accommodate only small substances suggests that this trafficking of large molecules may be a normal function. This work opens up possibilities for better understanding of plant growth and development and may provide better ways to use plants as sources of food, fiber and energy. This was a cooperative project among the plant biology section, plant pathology department and the Center for Engineering Plants for Resistance Against Pests. The research was funded by the U.S. Department of Agriculture Competitive Grants Program, the National Science Foundation and the UC Davis College of Agriculture and Environmental Sciences.